Freshwater Mussels in the South Branch

Freshwater mussels in the Chesapeake watershed seem much more vulnerable and are certainly less common than in the Ohio River drainage 40 miles west.  I'll bet 80% of those we do see are just empty shells, but I could tell these were alive from 15 feet away.  I've read that there were as many as 14 species surviving in the Potomac back in the 1970's.  Nowadays, fortunately, all are protected.